Well, I have partially changed mine a couple 2-3 times in five years. No drain plug, so you have to do the bit with tilting it over and balancing it on the milk jug to collect the old oil out the filler hole. I can only balance it like that so long, and I suspect not all is coming out, since it never takes the rated capacity to get back to the full mark. (Maybe 15 oz out of the 20 it says it takes.) Honda-engined red generic from one of the MTD brands, Yardman, I think. Gets maybe 60-75 hours use per year, depending on rain amounts.
So I suspect that if the mower burns or leaks oil, and you keep topping it off, you are doing about as much good as I did. This stuff I drain out is rather nasty looking. Now if I could just find a public drop-off site before the milk jugs disintegrate again. Good thing I keep them in a kitty litter pan now. (and no, you can't drain the mower into the kitty litter pan, without it spilling all over the mower.) It does start easier with fresh oil.